The known thermal over load relays are shown in FIGS. 7 and 8. The relay shown in FIG. 7 has a bimetallic strip 1, which buckles when an overcurrent flows through it. A shifter 2 that interlocks with the strip 1 moves to the left as viewed in the figure and pushes a release lever 3 to the left when the bimetallic strip buckles. Then, the release lever 3 rotates clockwise around the support pin 3a so that a protrusion 3b at the upper end of a release lever 3 is disengaged from an eccentric disk 9a connected to a set button 9. This makes the release lever 3 be pushed upward by a spring S. A projection 3c formed on the release lever 3 is pressed against a claw 5a attached to a movable plate 5 to angularly move the plate 5 in a counterclockwise direction about a groove 8a formed in a terminal 8. The movable contact 6 of the movable plate 5 then breaks contact with a normally-connected fixed contact 4 and makes contact with a normally-disconnected fixed contact 7. The movable plate 5 is urged into the groove 8a by a spring (not shown), so that it is electrically connected with the terminal 8. After the overcurrent is removed, by depressing a reset rod 10, a reset pin 10a pushes down the release lever 3 to disengage the projection 3c on the lever 3 from the claw 5a on the movable plate 5. The spring (not shown) rotates the movable plate 5 in a clockwise direction about the groove 8a until the protrusion 3b on the lever 3 engages the eccentric disk 9a. Thus, the reset operation is completed.
FIG. 8 schematically shows a change-over mechanism of another known relay. This relay includes a bimetallic strip 16 and a shifter 17. When the strip 16 buckles to move the shifter 17 to the left as viewed in the figure, the shifter 17 rotates a release lever 18 in a clockwise direction about a pivot X via a temperature-compensating bimetallic strip 18b. The free end of the lever 18 is in contact with a tension spring 14 that has an end engaged in a groove 15a formed in a terminal board 15. The other end of the spring 14 is fixed to a movable plate 11. When the free end of the lever 18 pushes the tension spring 14 past the position of the groove 15b in the terminal board 15 about which the movable plate 11 rotates, the movable plate 11 is quickly turned by the action of the tension spring 14. The movable contact point 12 of the movable contact 11 moves away from a normally connected fixed contact 13 and comes into contact with a normally-disconnected fixed contact (not shown).
In these conventional relays, the movable plates 5 and 11 are connected to the terminal boards 8 and 15, respectively. When electric wires are connected to the terminal boards 8 and 15, the tightening forces of terminal screws (not shown) may bring the grooves 8a and 15a out of position. If such a deviation occurs at 11, the change-over points of the boards shift to cause a change in the values of the operating current flowing through the thermal overload relays. Therefore, whenever electric wires are connected, it is necessary to readjust the movable and fixed contacts. These operations are cumbersome to perform and require a long time.